1. Technical Field
The invention is related to inductively coupled RF plasma reactors of the type having a reactor chamber ceiling overlying a workpiece being processed and an inductive coil antenna adjacent the ceiling.
2. Background Art
Inductively coupled RF plasma reactors are employed to perform a variety of processes on workpieces such as semiconductor wafers. Referring to FIG. 1, one type of inductively coupled RF plasma reactor has a reactor chamber 10 including a ceiling 12 and a cylindrical side wall 14. A pedestal 16 supports the workpiece 18, such as a semiconductor wafer, so that the workpiece generally lies in a workpiece support plane, and a bias RF power generator is coupled to the pedestal 16. A generally planar coil antenna 20 overlies the ceiling 12 and is coupled to a plasma source RF power generator 22. A chief advantage of inductively coupled RF plasma reactors over other types such as capacitively coupled ones, is that a higher ion density can be achieved with the inductively coupled type.
Adequate etch selectivity is achieved by operating at higher chamber pressure. (The term etch selectivity refers to the ratio of etch rates of two different materials exposed to etching in the reactor.) This is because the polymerization processes typically employed in a high density plasma etch reactor to protect underlying non-oxygen-containing (e.g., silicon, polysilicon or photoresist) layers during etching of an overlying oxygen-containing (e.g., silicon dioxide) layer are more efficient at higher chamber pressures (e.g., above about 20-500 mT) than at lower pressures. Polymer precursor gases (e.g., fluorocarbon or fluorohydrocarbon gases) in the chamber tend to polymerize strongly on non-oxygen-containing surfaces (such as silicon or photoresist), particularly at higher chamber pressures, and only weakly on oxygen-containing surfaces (such as silicon dioxide), so that the non-oxygen-containing surfaces are relatively well-protected from etching while oxygen-containing surfaces (such as silicon dioxide) are relatively unprotected and are etched. Such a polymerization process enhances the oxide-to-silicon etch selectivity better at higher chamber pressures because the polymerization rate is higher at higher pressures such as 100 mT. Therefore, it is desireable to operate at a relatively high chamber pressure when plasma-etching oxygen-containing layers over non-oxygen-containing layers. For example, under certain operating conditions such as a chamber pressure of 5 mT, an oxide-to-photoresist etch selectivity of less than 3:1 was obtained, and raising the pressure to the 50-mT range increased the selectivity to over 6:1. The oxide-to-polysilicon etch selectivity exhibited a similar behavior.
The problem with increasing the chamber pressure (in order to increase etch selectivity) is that plasma ion spatial density distribution across the wafer surface becomes less uniform. There are two reasons this occurs: (1) the electron mean free path in the plasma decreases with pressure; and (2) the inductive field skin depth in the plasma increases with pressure. How these two factors affect plasma ion spatial density distribution will now be explained.
With regard to item 1 above, the electron-to-neutral species elastic collision mean free path length, which is inversely proportional to chamber pressure, determines the extent to which electrons can avoid recombination with other gas particles and diffuse through the plasma to produce a more uniform electron and ion distribution in the chamber. Typically, electrons are not generated uniformly throughout the chamber (due, for example, to a non-uniform inductive antenna pattern) and electron diffusion through the plasma compensates for this and provides greater electron and plasma ion spatial density distribution uniformity. (Electron spatial density distribution across the wafer surface directly affects plasma ion spatial density distribution because plasma ions are produced by collisions of process gas particles with energetic electrons.) Increasing chamber pressure suppresses electron diffusion in the plasma, thereby reducing (degrading) plasma ion spatial density distribution uniformity.
This problem may be understood by reference to FIG. 1, in which the inductive antenna 20, due to its circular symmetry, has an antenna pattern (i.e., a spatial distribution of the magnitude of the induced electric field) with a null or local minimum along the antenna axis of symmetry so that very few if any electrons are produced over the wafer center. At low chamber pressures, electron diffusion into the space (xe2x80x9cgapxe2x80x9d) between the antenna 20 and the workpiece 18 is sufficient to transport electrons into the region near the wafer center despite the lack of electron production in that region, thereby providing a more uniform plasma distribution at the wafer surface. With increasing pressure, electron diffusion decreases and so plasma ion distribution becomes less uniform.
A related problem is that the overall plasma density is greater near the ceiling 12 (where the density of hot electrons is greatest) than at the workpiece 18, and falls off more rapidly away from the ceiling 12 as chamber pressure is increased. For example, the electron mean free path in an argon plasma with a mean electron temperature of 5 eV at a chamber pressure of 1 mT is on the order of 10 cm, at 10 mT it is 1.0 cm and at 100 mT it is 0.1 cm. Thus in a typical application, for a 5 cm ceiling-to-workpiece gap, most of the electrons generated near the ceiling 12 reach the workpiece at a chamber pressure of 1 mT (for a maximum ion density at the workpiece), and a significant number at 10 mT, while at 100 mT few do (for a minimal ion density at the workpiece). Accordingly, it may be said that a high pressure regime is one in which the mean free path length is about {fraction (1/10)} or more of the ceiling-to-workpiece gap. One way of increasing the overall plasma ion density at the workpiece 18 (in order to increase etch rate and reactor throughput) without decreasing the chamber pressure is to narrow the gap so that the mean free path length becomes a greater fraction of the gap. However, this exacerbates other problems created by increasing chamber pressure, as will be described further below.
With regard to item (2) above, the inductive field skin depth corresponds to the depth through the plasmaxe2x80x94measured downward from the ceiling 12xe2x80x94within which the inductive field of the antenna 20 is nearly completely absorbed. FIG. 2 illustrates how skin depth in an argon plasma increases with chamber pressure above a threshold pressure of about 0.003 mT (below which the skin depth is virtually constant over pressure). FIG. 2 also illustrates in the dashed-line curve how electron-to-neutral elastic collision mean free path length decreases linearly with increasing pressure. The skin depth function graphed in FIG. 2 assumes a source frequency of 2 MHz and an argon plasma density of 5xc2x71017 electrons/m3. (It should be noted that the corresponding plasma density for an electro-negative gas is less, so that the curve of FIG. 2 would be shifted upward with the introduction of an electro-negative gas.) The graph of FIG. 2 was derived using a collision cross-section for an electron temperature of 5 eV in argon. (It should be noted that with a molecular gas such as C2F6 instead of argon, the collision cross-section is greater so that the skin depth is greater at a given pressure and the entire curve of FIG. 2 is shifted upward.) If the chamber pressure is such that the inductive field is absorbed within a small fractionxe2x80x94e.g., {fraction (1/10)}thxe2x80x94of the ceiling-to-workpiece gap adjacent the ceiling 12 (corresponding to a pressure of 1 mT for a 5 cm gap in the example of FIG. 2), then electron diffusionxe2x80x94throughout the remaining {fraction (9/10)}ths of the gapxe2x80x94produces a more uniform plasma ion distribution at the workpiece surface. However, as pressure increases and skin depth increasesxe2x80x94e.g., beyond about {fraction (1/10)}th of the gap, then electron diffusion tends to have less effect. Thus, a measure of a high skin depth regime is that in which the skin depth is at about {fraction (1/10)} or more of the source-to-workpiece gap length. For example, if the pressure is so great that skin depth equals the ceiling-to-workpiece spacing (corresponding to a pressure of about 100 mT for a 5 cm gap in the example of FIG. 2), then any antenna pattern null or local minimum extends to the surface of the workpiece 18, effectively preventing electron diffusion from compensating for the effects of the antenna pattern null on the processing of the workpiece. Such problems can arise, for example, when the ceiling-to-workpiece spacing is decreased in order to increase overall plasma density at the workpiece surface. A related problem with a small ceiling-to-workpiece spacing and a high chamber pressure is that electrons are lost not only to recombination with particles in the processing gas but are also lost to recombination by collisions with the surface of the ceiling 12 and the workpiece 18, so that it is even more difficult for electrons generated in other regions to diffuse into the region adjacent the workpiece center.
In summary, plasma ion density at the wafer can be enhanced by reducing the gap between the axially symmetrical antenna/ceiling 20, 12 and the workpiece 18. But if the gap is reduced so much that the inductive field skin depth becomes a substantial fraction (xe2x89xa710%) of the gap, then ion density at the workpiece center falls off significantly relative to the edge due to the antenna pattern""s center null. However, for a smaller fraction of skin depth over gap and sufficient electron diffusion (characteristic of a low chamber pressure), electrons produced far from the workpiece center may diffuse into the center region before being lost to gas phase recombination or surface recombination, thereby compensating for the antenna pattern""s center null. But as the gap is reduced (to increase overall plasma density at the workpiece) and chamber pressure is increased (to enhance etch selectivity), then: (1) the induced electric field over the workpiece center approaches a null so that no electrons are produced in that region, and (2) electrons produced in other regions generally cannot diffuse to the workpiece center region due to recombination with gas particles and chamber (e.g., ceiling) surfaces.
Thus, as the wafer-to-coil distance is decreased by the reactor designer (in order to enhance plasma density near the wafer surface, for example), the plasma ion density decreases at the wafer center and ultimately, at very short wafer-to-antenna distances, becomes a center null giving rise to an unacceptable process non-uniformity. For example, in a plasma etch process carried out in such a reactor, the etch rate at the wafer center may be so much less than elsewhere that it becomes impossible to perform a complete etch across the entire wafer surface without over-etching near the wafer periphery. Conversely, it becomes impossible to avoid over-etching at the wafer periphery without under-etching the wafer center. Thus, the problem is to find a way to decrease the wafer-to-antenna distance without incurring a concomitant penalty in process non-uniformity.
One approach for solving or at least ameliorating this problem is disclosed in U.S. application Ser. No. 08/507,726 filed Jul. 26, 1995 by Kenneth S. Collins et al. and entitled xe2x80x9cPlasma Source with an Electronically Variable Density Profilexe2x80x9d, which discloses that an outer generally planar coil antenna 24 coupled to a second independently controlled plasma source RF power generator 26 can be provided over the ceiling 12 concentric with the inner coil antenna 20 of FIG. 1. The efficacy of this solution can be seen from the graphs of FIGS. 3A through 3E. FIG. 3A illustrates the plasma ion density as a function of radius from the center of the workpiece 18 for a workpiece-to-ceiling height of 4 inches (10 cm), the curve labelled A being the ion density produced by the outer coil antenna 24 and the curve labelled B being the ion density produced by the inner coil antenna 20. The total resulting plasma ion density is the sum of these two curves but is not depicted in the drawing for the sake of simplicity. FIG. 3A shows that at a height of 4 inches (10 cm), the outer coil antenna 24 produces a uniform plasma ion density distribution, the inner coil antenna 20 not being required. FIG. 3B corresponds to FIG. 3A for a reduced workpiece-to-ceiling height of 3 inches (7.5 cm), and shows that a dip in plasma ion density produced by the outer coil antenna 24 is compensated by the center-dominated ion density produced by the inner coil antenna 20. FIG. 3C corresponds to FIG. 3A for a further reduced workpiece-to-ceiling height of 2.5 inches (6.25 cm), and shows that the compensation by the inner coil 20 for the center dip in the plasma ion density produced by the outer coil 24 remains fairly effective as the workpiece-to-ceiling height is further reduced, although a slight dip in the total resulting plasma ion density near the center would begin to appear below this height. As shown in FIG. 3D, a further reduction in workpiece-to-ceiling height to only 1.25 inches (about 3.2 cm) yields a pronounced dip in the plasma ion densities produced by both the inner and outer coil antennas 20, 24, so that there is very little compensation and the resulting plasma ion density (the sum of the two curves shown) is highly non-uniform. As shown in FIG. 3E, the problem worsens as the height is further reduced to 0.8 inches (2 cm).
What FIGS. 3A-3E show is that even the use of inner and outer coil antennas to solve the problem of the null in plasma ion density near the workpiece center may lose effectiveness as the workpiece-to-ceiling height is reduced below certain values. Thus, the wafer-to-ceiling height cannot be reduced below a factor of the skin depth without sacrificing process uniformity. On the other hand, unless the wafer-to-ceiling height can be so reduced, plasma density and process performance is limited. Accordingly, there is a need for a way to reduce the workpiece-to-ceiling height without sacrificing process uniformity.
The invention is embodied in an inductively coupled RF plasma reactor including a reactor chamber enclosure defining a plasma reactor chamber and a support for holding a workpiece inside the chamber, a non-planar inductive antenna adjacent the reactor chamber enclosure, the non-planar inductive antenna including inductive elements spatially distributed in a non-planar manner relative to a plane of the workpiece, and a plasma source RF power supply coupled to the non-planar inductive antenna. Alternatively, the non-planar distribution of the antenna""s inductive elements is such that the inductive elements are spatially distributed approximately in respective planes intersecting the axis of symmetry. Although the inductive antenna may be either asymmetrical or symmetrical, the inductive antenna preferably includes a symmetrical solenoid winding such as a vertical stack of inductive windings. Generally, the invention provides a means for adjusting such processing parameters as plasma ion density distribution across the surface of the workpiece. More specifically, the invention compensates for a null in an RF inductive pattern of the antenna, which is typically near an axis of symmetry of the antenna. In order to accomplish this, in a preferred embodiment the windings are at a minimum radial distance from the axis of symmetry of the antenna so as to concentrate the induction field over the workpiece center for optimum process uniformity at small workpiece-to-antenna distances.
In an alternative embodiment, the windings are at a radial distance from the axis of symmetry which is a substantial fraction of a radius of the chamber. This radial distance is selected to be an optimum value which provides the greatest uniformity in plasma ion density under particular conditions which may include sources of process non-uniformities in addition to the antenna pattern center null. The determination of the optimum radial distance can be carried out by the skilled worker by trial and error steps of placing the solenoid winding at different radial locations and employing conventional techniques to determine the radial profile of the plasma ion density at each step.
For more versatility, the reactor may further include a second inductive antenna adjacent the reactor enclosure at an outer radial location relative to the solenoid winding and, preferably, a second plasma source RF power supply coupled to the second inductive antenna for independent adjustment of RF power applied to the inner and outer antennas. In one embodiment, the second inductive antenna is a second non-planar inductive antenna. In another embodiment, the second non-planar inductive antenna is a solenoid winding.
The reactor solenoid winding may be a doubly wound solenoid winding, which may consist of either a pair of concentric single solenoid windings or a vertical stack of pairs of windings. Likewise, if there is a second radially outward solenoid winding, then the second solenoid winding may be a doubly wound solenoid winding.
The vertical stack of conductive windings may have a right cylindrical shape, an upright conical shape or an inverted conical shape or a non-symmetrical shape. In order to distribute a selected portion of the induction field away from the center, a planar coil conductor may extend radially outwardly from a bottom winding of the vertical stack of conductive windings.
The invention is not confined to any particular non-planar configuration or shape, and any suitable shape can be employed which performs the function of providing the requisite concentration of the RF induction field near the center axis to compensate for the antenna pattern center null.